A passive optical network (PON) is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises. A PON consists of an Optical Line Termination (OLT) at the service provider's central office and a number of Optical Network Units (ONUs) also called Optical Network Terminals (ONTs) near end users. A PON configuration reduces the amount of fiber and central office equipment required, in comparison with point to point architectures. Downstream signals are broadcast to each premises sharing one and the same fiber. Upstream signals are combined using a multiple access protocol, invariably time division multiple access (TDMA). The OLTs “range” the ONUs in order to provide time slot assignments for upstream communication.
At least the following PON versions are widely known in the prior art: ATM PON (APON), Broadband PON (BPON), Ethernet PON (EPON).
The ITU-T G.984 (GPON) standard for a Gigabit PON is an evolution of the Broadband PON standard (BPON). GPON supports higher rates, enhanced security, total bandwidth and bandwidth efficiency and also choice of Layer 2 protocol (ATM, GEM, Ethernet) through the use of larger, variable-length packets. The standards permit several choices of bit rate, but the industry has converged on 2,488 Mbits per second (Mbit/s) of downstream bandwidth, and 1,244 Mbit/s of upstream bandwidth. The GPON Encapsulation Method (GEM) allows very efficient packaging of user traffic, which is a unique feature of GPON.
Treatment of so-called N:1 flows in access networks is defined in the DSL Forum TR101 [http://www.dslforum.org/techwork/tr/TR-101.pdf]. N:1 flows are understood as flows that belong to a single service and are forwarded from a single central unit such as DSLAM, OLT, etc. to N subscriber end devices such as CPEs, ONTs, etc. (the N:1 service may be, for example, a voice service, a video service, a digital service, etc.). According to the above approach, all ingress packets in OLT that belong to N:1 flows, are forwarded to the relevant port of the OLT (DSLAM, etc.) based on the information that was already learned and updated in a Forwarding Information Base (FIB) of the OLT (DSLAM). The FIB information associates the packet's VLAN-ID (ID in a Virtual Local Area Network) and a Destination MAC address (Media Access Control address) with the relevant Port of OLT (DSLAM).
In the GPON case, a Port of OLT is understood as a combination of a physical PON port and the matching GEM port ID, wherein a GEM port ID indicates a specific ONT associated with the specific physical PON port and being a member in an N:1 service group that comprises N end users.
Unlike in a regular DSL case where the OLT port constitutes a physical DSL port, in the GPON case the GEM port ID is a virtual port, and several such virtual ports can reside in the same physical PON port.
In access networks, treatment of packets with unknown destination is usually organized as follows. In case the packet's combination of VLAN-ID+Destination MAC is not included in the FIB table (i.e., it has not been learned yet), that packet of the downstream flow is forwarded/flooded to all the ports. Actually, packets with unknown destination can be understood and handled as broadcast packets.
In a regular DSL case, a packet with unknown destination will be flooded (broadcast) to all relevant DSL ports, and then to all respective CPEs that are members in a specific N:1 service.
In a regular GPON case, a packet with unknown destination arriving to a physical port connected to its associated optical network (PON), will be flooded (broadcast) to all relevant GEM port IDs (i.e., to all ONTs, CPEs, MAC bridge ports, etc. that are members in the relevant N:1 service). According to that conventional solution, the packet will be transported N times per PON (where N is the number, of relevant GEM ports, and also the number of respective ONTs being members in that N:1 service). The described arrangement is shown in FIG. 1 (prior art). Note that a packet issued for any specific GEM port will reach all ONTs etc. connected to the physical port due to the broadcast nature of the PON. However, each virtual GEM port will be terminated (i.e., the forwarded packets will be accepted) by only one member in the ONT which is the required destination for a packet sent from that GEM.
Naturally, the above-described arrangement results in a significant bandwidth (BW) loss in the case of unknown packet destination, since all GEM ports carry the same flooded information.
The problem of reducing BW loss and reducing the amount of flooded packets in PONS is being studied in the prior art.
U.S. Pat. No. 6,967,949 describes a technique for forwarding packets in an Ethernet Passive Optical Network (EPON). The OLT unit in an EPON network may flood a packet with an unknown MAC destination address to all user-side ports within a given broadcast domain. If such a flooding is to be done, it will be performed via all individual user-side OLT ports belonging to the broadcast domain i.e., in a way similar to the regular DSL case.
Since GPON does not have the Ethernet physical layer, but uses the GPON Encapsulation Method instead, the described EPON solution is not applicable to GPON.